The present invention generally relates to a line image sensor which is used in an image signal portion for a copying machine, a FAX, an image scanner or the like.
In recent years, the demand for a higher copy speed and lower price in copying machines has increased. The demand for a higher copy speed and lower price in the line image sensors used in copying machines has also increased.
The long-standing line image sensor is composed of: a light receiving portion for converting the incident light into electrical signals with light receiving elements being arranged in a main-scanning direction: a transfer portion for receiving electrical charge caused in the above described light receiving elements so as to transfer it; an output portion having two output portions for receiving the electrical charge from the transfer portion and for converting it into an output voltage signal. (Tompsett ets. "Charge Coupled Imaging Devices: Experimental Result, "IEEE Trans. on Electron Devices ED-18, 992-996 (1971))
Signals generated in the light receiving portion consisting of N light receiving elements are r transferred as 94 electrical charge into the transfer portion. In the transfer portion, the signal electrical charge is sequentially transferred towards the output portion. In the output portion, N serial data are outputted.
The output portion cannot convert the volume of electrical charges to a voltage signal when the driving frequency is high. Therefore, the line image sensor cannot be driven at high speeds.
In order to compensate for the defect, there is also a line image sensor, which is bilinear in construction, and which is composed of two output portions, pairing off each transfer portion with the transfer portion being disposed on both the side faces of the light receiving portion. (Tompsett etc. "Charge Coupling Improves Its Image, Challenging Video Camera Tubes, "Electronics 46, No. 2, 162-168 (1973)).
In this case, signals generated in a light receiving portion composed of N of light receiving elements are alternately transferred into the transfer portion at both side faces. In each transfer portion, the N/2 data is transferred. Similarly, two output portions each handle half the data being processed, so that a data rate which is twice the driving frequency may be obtained.
In the above described construction, the width of the line image sensor becomes larger. Especially, in the case of a color image sensor made with three lines thereof being arranged in parallel in the sub-scanning direction, the clearance of the light receiving portion is larger, and the correction memory required for the positional alignment of each image sensor is necessarily large in size. Assume that the clearance of the light receiving portion has 12 line pitches, and the information of a point before 12 lines is read by a first sensor before the next light receiving portion reads the information of that time after the information at a certain point has been read by a first light receiving portion. The memory amount which is necessary in the reading A3 manuscript at 400DPI, eight gradations becomes: EQU 5000 image elements.times.(13+26)=195 k byte